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Heavy front vibration problem - Torsion Bars
https://policeinterceptor.com/revconforums/viewtopic.php?f=14&t=207
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Author:  Austro-Revconeer [ Sun Feb 06, 2022 4:45 pm ]
Post subject:  Heavy front vibration problem - Torsion Bars

The former owner of my Revcon had this problem since he bought it from the first owner in 1998. Under 58 mph it´s no big deal. I can only feel the vibrations on the accelerator pedal. But when you go faster the whole dashboard in front of you starts shaking very badly - it even tossed my attached smartphone out of the holder a couple of times. In that case I have to reduce the speed immediately. Very annoying when passing a truck on the Autobahn.

All parts of the front suspension are pretty new, the former owner replaced them step by step in order to get rid of the strange vibrations but all his efforts were useless. The only point what is now left are the Torsion Bars.

We also balanced the front wheels in July 2021 - no improvement. It was not that easy to find a shop in Germany who was willing to do the work. It´s the pure size of the Revcon what the mechanics here in Europe are afraid of - no joke. I have experienced this multiple times. :?

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Wheel Balancing R. Forum.jpg
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Next step was that we turned the adjusting bolts and the front went some way up, but still didn´t reach the correct riding height. Big surpise: The vibrations up to 58 were not so heavy anymore (!), a clear sign that the whole problem has to do with the Torsion Bars somehow. But in September it suddenly went "Crack!" and since this time the adjustment bolt on the driver´s side shows a weird angle.

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Broken Adjusting Bolt.jpg
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From that time on the ride was absolutely rough too. I´m sure that the front suspension doesn´t work anymore. Right after that incident I parked the coach under the flying roof and prepared it for the winter time. Well, it lasted for 42 years. The former owner did all vacation trips with that problem in 20 years.

So here is the plan: I´m going to order all the parts from Applied GMC this spring. My UPP Oldsmobile Toronado FWD is similar with the one from the GMC Motorhome.

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https://www.gmcrvparts.com/

Author:  Austro-Revconeer [ Sun Feb 06, 2022 5:00 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

I would prefer new ones instead of used Torsion Bars, but I haven´t found any yet.

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Good Used Bars R. Forum.jpg
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Two Torsion Bar Special Nut Blocks, I have already bought 2 Longer Torsion Bars Adjusting Bolts from Applied GMC, but they didn´t fit in my Nut Blocks. 2 Torsion Bar Anchor Arms and one Torsion Bar Unloader, probably the AP45, because I have found a PDF File on the net (GMC Motorhome 1976 Maintenance Manual Supplement) which shows you how to position the Unloader Tool.

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Longer Adjust. Bolts R. Forum.jpg
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Author:  Austro-Revconeer [ Sun Feb 06, 2022 5:05 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

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Unloading Tool Pic R. Forum.jpg
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A German member from the Astro/Safari Forum gave me some information about how to replace the Torison Bars:

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Author:  Austro-Revconeer [ Sun Feb 06, 2022 5:06 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

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Author:  Austro-Revconeer [ Sun Feb 06, 2022 5:12 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

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Well, let´s hope that this will work out fine. Please don´t forget that I´m only a litte Austrian U.S. Car enthusiast without any mechanical apprenticeship. ;)

I will keep you current.

Any information concerning this topic is highly appreciated! Thank you.
Take care!
Matt

Author:  Austro-Revconeer [ Sat Feb 12, 2022 12:46 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

What else could cause that vibrations? I got some information from the GMC Motorhome Forum.

wheel balance - checked
drive axles - replaced
According to information of the former owner of my Revcon, who is a restorer of U.S. military vehicles in Germany and who ordered drive axles for a 1978 Oldsmobile Toronado which fit for the Revcon from Rock Auto and changed them twice in 20 years. I drove without the engine cover in July 2021 and watched the engine and the drive axles closly from above. I didn´t see any abnormalities.
Diff oil - new in 2018
front brake discs - new in 2018, rear drum brakes too.

The specialists in the GMC Motorhome forum are wondering if the torsion bars can really be responsible for the vibrations.

At 58 mph it starts and it´s not only a kind of vibration, I would say a little more specific that it´s a really bad shaking, the whole dashboard and the steering rod wobbles and it feels like the front end begins to fall apart. It also depends on the condition of the road. I did 80 mph several times without heavy shaking illegally during a 320 miles ride when there was an even road surface (the speed limit on German Autobahns for vehicles exceeding 3.5 tons is 100 km/h - 62 mph).

Here is the picture of a Revcon with good torsion bars and correct riding height.

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1980ies Side View Revcon R. Forum.jpg
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There is definitely a difference.

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Revcon Side View July 2020 R. Forum.jpg
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Interesting:

"Torsion bar suspensions are used on combat vehicles and tanks like the T-72, Leopard 1, Leopard 2, M26 Pershing, M18 Hellcat, and the M1 Abrams (many tanks from World War II used this suspension), and on modern trucks and SUVs from Ford, Chrysler, GM, Mitsubishi, Mazda, Nissan, Isuzu, LuAZ, and Toyota. Manufacturers change the torsion bar or key to adjust the ride height, usually to compensate for engine weight. While the ride height may be adjusted by turning the adjuster bolts on the stock torsion key, rotating the stock key too far can bend the adjusting bolt and place the shock piston outside its standard travel. Over-rotating the torsion bars can also cause the suspension to hit the bump-stop prematurely, causing a harsh ride."

Source: https://en.wikipedia.org/wiki/Torsion_bar_suspension

Anytime when there is a bump on the road it hits the coach and it feels like there is no front shock absorbing, but the shocks are new too. It´s getting worse since the adjusting bolt has got its weird angle.

Take care,
Matt

Author:  Austro-Revconeer [ Wed Feb 16, 2022 3:42 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

Info from the GMC Motorhome forum:

If you are down on the bump-stops and you can't adjust to be off them, then you have to change something.

Well, so let's order the parts.

Does anybody know where I could get new Torsion Bars? I have read somewhere that a company started to produce new "improved" ones for the GMC Motorhome some years ago, but only for a certain time. Applied GMC offers "good used bars", but no new ones.

Are there any sources for original Revcon Torsion Bars available?

Would be good to know.
Thanks.
Matt

Author:  tttphd [ Sat Feb 19, 2022 8:51 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

A few things I would check are (1) the supports on the transfer gear, (2) the supports on the differential, (3) the universal joints on the drive shaft and half shafts, and (4) the differential bearings for loosness. Of course, I am as ssuming that yours is like my 1984 King.

In addition, if the front wheel drive is like mine, you could raise the front end and using stationary supports under both A arms, run the vehicle and observe what is happening. I did this on mine and it worked fine. Just be sure to put support under the A arms to keep the wheels off the ground.

Author:  Daveinet [ Tue Feb 22, 2022 1:13 am ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

The only thing that could cause vibration due to torsion bars would be if the ride height was low enough to cause the C-V joints to bind. if the shafts are straight, then the torsion bars are not the cause. You could have any number of things wrong. As stated above, use floor jacks under the A-arms and lift the tires off the ground. Run it up to speed in the air so you can see what vibrates, or view anything that is out of round.

Author:  Austro-Revconeer [ Thu Feb 24, 2022 2:22 pm ]
Post subject:  Re: Heavy front vibration problem - Torsion Bars

Thank you for your suggestions!

@tttphd: O.K. - Numbers 1 to 4 are on my list.
@Yes Dave, I´m going to check this in April.

Right now, the Revcon is stored in its winter parking lot in Germany.

Matt

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